1. Field of the Invention
The present invention relates to a method for erasing and verifying a nonvolatile semiconductor memory device.
2. Description of the Related Art
In a nonvolatile semiconductor memory device, an electrical write operation can be carried out after the device is mounted on a printed circuit board. One typical example of such a device is a flash memory device which has an advantage in that data storage is possible without a backup battery, and which is highly integrated.
In a first prior art erasing and verifying method for a flash memory device, in order to suppress the deviation of threshold voltage after a flash erase operation, a write operation is performed upon memory cells before the erase operation. This will be explained later in detail.
In the first prior art erasing and verifying method, however, when some of the memory cells are overerased, i.e., enter a depletion state, it is impossible to compensate for this overerasing. Note that the depletion state memory cells invite a malfunction of write/read operation.
In a second prior art erasing and verifying method, a flash overerase operation after a flash erase operation is performed upon all of the memory cells, so that all of the memory cells are of a depletion type. Then, a threshold voltage recovering operation, which is a kind of write operation, is performed upon the memory cells, to suppress the deviation of threshold voltage as well as avoid the generation of depletion state memory cells (see: K. Oyama et al., "A Novel Erasing Technology for 3.3 V Flash Memory with 64 Mb Capacity and Beyond", IEDM Technical Digest, 1992, pp. 607-610; and S. Yamada et al., "A SELF-CONVERGENCE ERASING SCHEME FOR A SIMPLE STACKED GATE FLASH MEMORY", IEDM Technical Digest, 1991 pp. 307-310). This will be explained later in detail.
In the second prior art erasing and verifying method, however, in spite of the enhancement/depletion state of the memory cells, a flash overerase operation for putting the memory cells in a depletion state and a threshold voltage recovering operation are carried out, to increase the time required for erasing and verifying the flash memory device.